Grating coupler and integrated grating coupler system

ABSTRACT

A grating coupler having first and second ends for coupling a light beam to a waveguide of a chip includes a substrate configured to receive the light beam from the first end and transmit the light beam through the second end, the substrate having a first refractive index n1, a grating structure having curved grating lines arranged on the substrate, the grating structure having a second refractive index n1, wherein the curved grating lines have line width w and height d and are arranged by a pitch Λ, wherein the second refractive index n2 is less than first refractive index n1, and a cladding layer configured to cover the grating structure, wherein the cladding layer has a third refractive index n3. The curves of the grating lines are constructed such that the emitting beam is shaped for efficient coupling to another optical component. The curves can also be tilted to reduce coupling back into the waveguide.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority benefit of U.S. Provisional PatentApplication Ser. No. 62/924,287, filed on Oct. 22, 2019, the contents ofwhich are incorporated herein by reference in their entireties and thebenefits of each are fully claimed.

FIELD OF THE INVENTION

This invention generally relates to grating couplers for optical chips,also called photonic integrated circuits (PICs), and more particularlyto grating coupler system connecting one active chip and one passiveoptical chip.

BACKGROUND OF THE INVENTION

The target application is a hybrid integration of an active optical chipsuch as one containing InP waveguides and a passive chip such as onecontaining silicon and/or silicon nitride waveguides. The expectedproperties are, large tolerance to mis-alignment, easiness of bondingprocesses, and high coupling efficiency.

Silicon photonics offer many advantages of which the fabrication cost isthe most important factor. Furthermore, high refractive index contrastbetween the silicon waveguide and the surrounding silicon dioxide layersoffer tight bending with low loss possible, leading to higher densityand complexity PICs. Silicon nitride waveguides offer similar low costcapabilities, with lower optical loss property. On the other hand, thereis no reliable optical gain or emission capability with direct currentinjection. Therefore, hybrid integration of active PICs (such as InP,GaAs, or GaN-based ones) with passive silicon photonics PICs become veryimportant to achieve low cost, full functionality, and high densityPICs.

However, optically connecting two waveguides precise requires precisealignment typically with sub-micron accuracy, due to narrow waveguidesand thus fast diverging beam on both sides. There is a need to connecttwo optical chips with larger tolerance with high coupling efficiency.

SUMMARY OF THE INVENTION

Some embodiments of the present disclosure are based on recognition thattwo-dimensional long period grating on a passive waveguide from anoptical chip creates shallow angle emission towards the substrate side,diffracted at the chip facet (second end) at a steeper angle,manipulated to form a narrow beam, and then coupled to the passiveoptical chip through a grating coupler.

In accordance to some embodiments, a novel grating coupler system isrealized by a grating coupler having first and second ends for couplinga light beam to a waveguide of a chip including a substrate configuredto receive the light beam from the first end and transmit the light beamthrough the second end, the substrate having a first refractive indexn1; a grating structure having grating curves (lines) arranged on thesubstrate, the grating structure having a second refractive index n2,wherein the grating curves (lines) have line width w and height d andare arranged by a pitch Λ, wherein the second refractive index n2 isgreater than first refractive index n1; and a cladding layer configuredto cover the grating structure, wherein the cladding layer has a thirdrefractive index n3, wherein the third refractive index n3 is differentfrom the second refractive index n2, wherein the cladding layer isarranged so as to reflect the light beam diffracted from the gratingstructure toward below the cladding layer. The two-dimensional gratingcurves (lines) comprise a series of arcs which are part of ellipselines, whose pitch is gradually decreased in two dimensions, such thatthe diffracted beam is shaped or narrowed to have a focused spot on thesecond grating, which is typically a silicon grating.

In accordance with another embodiment of the present invention, agrating coupler having first and second ends for coupling a light beamto a waveguide of a chip includes a substrate configured to receive thelight beam from the first end and transmit the light beam through thesecond end, the substrate having a first refractive index n1; a gratingstructure having grating curves arranged on the substrate, the gratingstructure having a second refractive index n2, wherein the gratingcurves have line width w and height d and are arranged by a pitch Λ,wherein the second refractive index n2 is greater than the firstrefractive index n1, wherein the grating curves are arranged to diffractthe light beam to form a narrowing beam in a two orthogonal axesperpendicular to a light propagation direction of the light beam; and acladding layer configured to cover the grating structure, wherein thecladding layer has a third refractive index n3, wherein the thirdrefractive index n3 is different from the second refractive index n2.

Further, another embodiment of the present invention is based onrecognition that an integrated grating coupler system includes a gratingcoupler formed on a first chip, the grating coupler having first andsecond ends for coupling a light beam to a waveguide of a second chip,wherein the grating coupler comprises a substrate configured to receivethe light beam from the first end and transmit the light beam throughthe second end, the substrate having a first refractive index n1; agrating structure constructed to have grating curves arranged on thesubstrate, the grating structure having a second refractive index n2,wherein the grating curves have line width w and height d and arearranged by a pitch Λ, wherein the second refractive index n2 is greaterthan first refractive index n1; and a cladding layer configured to coverthe grating structure, wherein the cladding layer has a third refractiveindex n3, wherein the third refractive index n3 is less than the secondrefractive index n2. The cladding layer can be of the same material asthe substrate, or SiO₂, Si₃N₄, or polymer.

BRIEF DESCRIPTION OF THE DRAWINGS

The presently disclosed embodiments will be further explained withreference to the attached drawings. The drawings shown are notnecessarily to scale, with emphasis instead generally being placed uponillustrating the principles of the presently disclosed embodiments.

FIG. 1 shows a cross-sectional view of an integrated grating couplersystem according to embodiments of the present invention;

FIG. 2A shows the top view of the two-dimensional grating curves whichincludes the thickness of each grating line, according to embodiments ofthe present invention;

FIG. 2B shows the top view of center lines of the two-dimensionalgrating curves to embodiments of the present invention;

FIG. 3 shows a two-dimensional grating structure, where the grating isarranged to be asymmetric with respect to the light propagationdirection of the light beam such that the reflected light from thegrating curves is prevented from coupling to the first end of thewaveguide, according to embodiments of the present invention;

FIG. 4 shows a side view of the multi-step grating structure, accordingto embodiments of the present invention;

FIG. 5 shows a side view of the gratings wherein the asymmetric gratingsare formed by multiple steps, according to embodiments of the presentinvention;

FIG. 6A show a cross-section view of an example structure a gratingcoupler including a first chip (light beam transmission side) and asecond chip (receiving the transmitted beam from the first chip),according to embodiments of the present invention; and

FIG. 6B shows the top view of the grating coupler system of FIG. 6A,according to embodiments of the present invention.

While the above-identified drawings set forth presently disclosedembodiments, other embodiments are also contemplated, as noted in thediscussion. This disclosure presents illustrative embodiments by way ofrepresentation and not limitation. Numerous other modifications andembodiments can be devised by those skilled in the art which fall withinthe scope and spirit of the principles of the presently disclosedembodiments.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description provides exemplary embodiments only, and isnot intended to limit the scope, applicability, or configuration of thedisclosure. Rather, the following description of the exemplaryembodiments will provide those skilled in the art with an enablingdescription for implementing one or more exemplary embodiments.Contemplated are various changes that may be made in the function andarrangement of elements without departing from the spirit and scope ofthe subject matter disclosed as set forth in the appended claims.

Specific details are given in the following description to provide athorough understanding of the embodiments. However, understood by one ofordinary skill in the art can be that the embodiments may be practicedwithout these specific details. For example, systems, processes, andother elements in the subject matter disclosed may be shown ascomponents in block diagram form in order not to obscure the embodimentsin unnecessary detail. In other instances, well-known processes,structures, and techniques may be shown without unnecessary detail inorder to avoid obscuring the embodiments. Further, like referencenumbers and designations in the various drawings indicated likeelements.

Furthermore, embodiments of the subject matter disclosed may beimplemented, by use of at least in part, or combinations of parts of thestructures described below.

Optical coupling between two optical chips constitute the most importantpart of hybrid PICs. The easiness of alignment and high couplingefficiency are very important factors. Grating couplers offer thesecapabilities. In some cases, conventional elliptic grating curves createa collimating beam, i.e., beam shape is almost constant along thepropagation axis. However, this is not sufficient when the emission areais large and narrower beam width, or focusing, is necessary to coupleinto the second grating efficiently. According to embodiments of thepresent invention, it provides shapings of grating curves, such that thebeam is formed to be of a desired shape at the surface of the secondgrating, resulting in higher coupling efficiency.

There are multiple factors in achieving high coupling efficiency forthis configuration.

FIG. 1 shows a cross-sectional view of an integrated grating couplersystem 100 according to the invention. The first optical chip (firstchip) 105 is made on an InP substrate 110, containing an InGaAsPwaveguide layer 130, InP cladding layer 120, and a first grating 140.The second optical chip (second chip) 145 comprises of a siliconsubstrate 150, a buried SiO₂ layer (also called a BOX layer) 160, asilicon (Si) waveguide layer (also called silicon-on-insulator, or SOI)170, and a SiO₂ cladding layer 180, and the second grating 190 etchedonto the silicon waveguide layer. The diffracted light in the firstoptical chip 105 propagates through the InP substrate 110 and the firstoptical chip facet 195, and is coupled into the grating 190 on thesecond optical chip 145. Further, an example of a top view of anintegrated grating coupler system is shown in FIGS. 6A and 6B.

Here, the grating pitch A is the distance between the rising edges ofthe grating, w is the line width of the main tooth, and d is thethickness of the grating. The grating pitch Λ does not have to beconstant, and can be a function of the propagation distance from the endof the input waveguide, expressing a chirped grating. The grating pitchΛ also depends on the angle from the primary propagation distance, toform elliptic lines. In the first optical chip, grating diffracts lighttowards the substrate as a shallow angle, which is further diffracted atthe chip fact to a steeper angle. The beam is shaped and is shone on thegrating in the second chip and is guided to its waveguide. The operatingwavelength of 1530-1570 nm, the typical grating pitch Λ is 5-15 μm, andthe typical grating line width w is 10-60% of the grating pitch,depending on whether sub-gratings are included, or how the sub-gratingsare designed. The typical grating thickness d is 0.2-1 μm.

FIG. 2A shows an example illustrating a top view of a grating structure295, wherein the shaded region 240 is the area whose cladding layerthickness is greater than the surrounding area, 220 is the etchedgrating regions, and 230 is the input waveguide.

It should be noted that a distance L_(gr) between a straight end of thefirst end to a first grating line is arranged so that substantial amountof the intensity of the light beam can reach to the first grating curve(line) without unwanted diffractions of the light beam. For instance,the distance L_(gr) may be a range of nλ_(g) (n: a multiplier; λ_(g):wavelength of the light beam in the waveguide), where the multiplier nmay be between 10 to 1000, more preferably, 50-500.

FIG. 2B shows a top view of center lines of the grating curves of theetched regions 220, wherein the grating curves are expressed as

$\begin{matrix}{{q\;\lambda} = {{{xn}_{c}\mspace{14mu}\cos\mspace{14mu}\phi_{c}} - {n_{eff}\left( {x^{2} + y^{2}} \right)}^{\frac{1}{2}} + {\Delta_{x}x^{2}} + {\Delta_{y}y^{2}}}} & (1)\end{matrix}$where x and y are the directions parallel to and perpendicular to thelight propagation in the grating structure, respectively, q=m, m+1, m+2. . . (m>0) is the integer corresponding to each grating line, λ is thewavelength, n_(c) is the refractive index of the substrate, ϕ_(c) is theangle from the waveguide surface normal, n_(eff) is the effectiverefractive index of the waveguide, Δ_(x) and Δ_(y) are the coefficientsof grating chirp, expressing the narrowing or focusing effect in x and ydirection. Negative values of Δ_(x) and Δ_(y) mean that the pitch orspacing of the curves decrease as the curves move away from the origin(0, 0), i.e., the end of the input waveguide. Note that Eq. (1) does notnecessarily express ellipse lines unless both Δ_(x) and Δ_(y) are zero,however, they can be very well approximated by ellipse lines. The actualgrating curves are part of the Eq. (1), such that they form protrusionstoward a light propagation direction of the light beam as shown in FIG.2A.

The diffracted light from this grating can be manipulated in twodimensions, i.e., in the two orthogonal axes each perpendicular to thediffracted beam propagation direction. With a proper choice of thenegative values for Δ_(x) and Δ_(y), the diffracted beam can be narrowedas it propagates. In the case where Δ_(x) or Δ_(y) is equal to zero,i.e., the distance between the grating curves stays constant, thediffracted beam stays collimated in the corresponding direction.

Also, when they the absolute values of Δ_(x) and Δ_(y) are small, thengrating curves expressed by Eq. (1) have distances decreasing at a fixedrate. This value may be determined, typically between 0.2% and 2% of thepitch, so as to have enough narrowing effect (to form a narrowing beamwithin the area of the grating 640, see FIGS. 6A and 6B) but not to havetoo close focusing distance.

An integrated grating coupler system may contain semiconductor lasers onthe same substrate, however, semiconductor lasers are very sensitive toany reflection. It may cause mode hopping or laser linewidthfluctuation. Therefore, it is very important to minimize any reflectionfrom the optical components inside or outside of the cavity, includinggrating couplers which tend to show small amount of back reflection.

FIG. 3 shows a schematic of a two-dimensional grating, where thetwo-dimensional grating is asymmetric with respect to the lightpropagation direction of the light beam 310. In this case, thetwo-dimensional grating is arranged so that the reflected light from thegratings curves is prevented from coupling to the first end of thewaveguide. In other words, the axes 320 of the curves 300 (long axes inthe case of nearly ellipse curves) cross the waveguide line 310 withnon-zero angle α₀.

The cladding layer can be a non-semiconductor material. Contrary tousing semiconductor cladding layer which usually requires costly crystalregrowth, dielectric (SiO₂ or Si₃N₄) or polymer materials do not requireregrowth, so the fabrication is easier and cost is lower.

However, the refractive index of dielectric or polymer materials aretypically between 1.4 and 2.3, while that of the waveguide layer isbetween 3.0 and 3.6 at the wavelength of 1.3-1.6 μm where most opticalcommunications take place. Therefore, the refractive index differencebetween the waveguide layer and the cladding layer becomes larger thanwhen a semiconductor is used in the cladding layer. This creates asituation where higher-order (n=2, 3, 4, 5 . . . ) diffraction players alarger role, and reduces the coupling efficiency to another gratingcoupler (or another optical component), typically made on Si substrate.Therefore, it is very important to minimize the higher-orderdiffraction.

Each order of diffraction is highly correlated to the Fourier componentof the diffraction grating. For example, rectangular diffraction gratingcontains large amount of third-order and fifth-order Fourier component,so the third- and fifth order diffraction is very high. Therefore, it isimportant to effectively soften the rising and falling edge of thegrating.

FIG. 4 shows the side view of the gratings, wherein the waveguide layer420, sandwiched by a substrate 410 and a cladding layer 430, is formedby a grating 460 with more than two height levels or steps. This can beformed by multiple photolithography and etching processes. Since thetypical grating pitch for InP grating couplers is 8-12 μm, the formationof the multiple-step grating is feasible even with processes capable theminimum feature size of −0.5 μm.

In addition, the cross-sectional shape of the grating can be asymmetricas shown in FIG. 5. With respect to the direction of the lightpropagation 540, the grating can have a sharper sizing etch and slowerfalling edge, which creates an effective blazing grating effect. Thisway, the input light is more effectively directed to the downwarddirection 550.

FIG. 6A show a cross-section view of an example structure a gratingcoupler including a first chip (light beam transmission side) and asecond chip (receiving the transmitted beam from the first chip). Asdescribed above the grating lines are arranged according to Equation (1)with respect to FIGS. 2A and 2B, having predetermined distances andcurvatures. FIG. 6B shows the top view of the grating coupler system ofFIG. 6A, where the grating lines are curved. θ₁ and θ₂ are the anglesfor the concentric grating lines for the first and second chips,respectively. One way to narrow the lateral beam divergence is to usecurved gratings, such as elliptic grating. FIGS. 6A and 6B show across-sectional view and a top of the grating coupler system 600,respectively, wherein the first optical chip 610 and second optical chip630 have elliptical gratings 620 and 640, respectively. In one example,an InP waveguide 615 with around 1 μm width is connected to the ellipticgrating 620 with at least 10° in full width. A silicon waveguide 635with 0.5 μm width is also connected to an elliptic silicon grating 640.

The above-described embodiments of the present invention can beimplemented in any of numerous ways. For example, the embodiments may beimplemented using hardware, software or a combination thereof. Whenimplemented in software, the software code can be executed on anysuitable processor or collection of processors, whether provided in asingle computer or distributed among multiple computers. Such processorsmay be implemented as integrated circuits, with one or more processorsin an integrated circuit component. Though, a processor may beimplemented using circuitry in any suitable format.

Also, the embodiments of the invention may be embodied as a method, ofwhich an example has been provided. The acts performed as part of themethod may be ordered in any suitable way. Accordingly, embodiments maybe constructed in which acts are performed in an order different thanillustrated, which may include performing some acts simultaneously, eventhough shown as sequential acts in illustrative embodiments.

Use of ordinal terms such as “first,” “second,” in the claims to modifya claim element does not by itself connote any priority, precedence, ororder of one claim element over another or the temporal order in whichacts of a method are performed, but are used merely as labels todistinguish one claim element having a certain name from another elementhaving a same name (but for use of the ordinal term) to distinguish theclaim elements.

Although the present disclosure has been described with reference tocertain preferred embodiments, it is to be understood that various otheradaptations and modifications can be made within the spirit and scope ofthe present disclosure. Therefore, it is the aspect of the append claimsto cover all such variations and modifications as come within the truespirit and scope of the present disclosure.

What is claimed is:
 1. A grating coupler having first and second endsfor coupling a light beam to a waveguide of a chip comprising: asubstrate configured to receive the light beam from the first end andtransmit the light beam through the second end, the substrate having afirst refractive index n1; a grating structure having grating curvesarranged on the substrate, the grating structure having a secondrefractive index n2, wherein the grating curves have line width w andheight d and are arranged by a pitch Λ, wherein the second refractiveindex n2 is greater than the first refractive index n1, wherein thegrating curves are arranged to diffract the light beam to form anarrowing beam in a two orthogonal axes perpendicular to a lightpropagation direction of the light beam; and a cladding layer configuredto cover the grating structure, wherein the cladding layer has a thirdrefractive index n3, wherein the third refractive index n3 is differentfrom the second refractive index n2.
 2. The grating coupler of claim 1,wherein curves of the grating curves are constructed such that thediffracted beam is shaped for coupling to another grating coupler. 3.The grating coupler of claim 1, wherein curves of the grating curves areconstructed such that the diffracted beam is focused on another gratingcoupler.
 4. The grating coupler of claim 1, wherein the grating curvesare arranged as partial elliptic lines such that the partial ellipticlines form curves having protrusions toward a light propagationdirection of the light beam, wherein the spacing between the lines arenarrowed as a function of a distance from the end of a waveguide.
 5. Thegrating coupler of claim 1, wherein the center of the grating curves areexpressed as${q\;\lambda} = {{{xn}_{c}\mspace{14mu}\cos\mspace{14mu}\phi_{c}} - {n_{eff}\left( {x^{2} + y^{2}} \right)}^{\frac{1}{2}} + {\Delta_{x}x^{2}} + {\Delta_{y}y^{2}}}$where x and y are directions parallel to and perpendicular to the lightpropagation, respectively, wherein q=m, m+1, m+2 . . . (m>0) is aninteger corresponding to each grating line from the first end, λ is awavelength of the light beam, n_(c) is a refractive index of thesubstrate, ϕ_(c) is an angle from the waveguide surface normal, n_(eff)is an effective refractive index of the waveguide, Δ_(x) and Δ_(y) arecoefficients of grating chirp.
 6. The grating coupler of claim 1,wherein the grating curves are arranged in an asymmetric manner withrespect to a light propagation direction of the light beam, such thatthe reflected light from the grating curves is prevented from couplingto the first end of the waveguide.
 7. The grating coupler of claim 1,wherein the cladding layer comprises silicon dioxide.
 8. The gratingcoupler of claim 1, wherein the cladding layer comprises siliconnitride.
 9. The grating coupler of claim 1, wherein the cladding layercomprises polymer.
 10. The grating coupler of claim 1, wherein thecladding layer comprises the same material as the substrate.
 11. Thegrating coupler of claim 1, wherein the grating comprises more than twoheight levels.
 12. The grating coupler of claim 1, wherein across-sectional shape of the grating is asymmetric.
 13. The gratingcoupler of claim 11, wherein a rising edge of the grating is shaper thana falling edge of the grating.
 14. The grating coupler of claim 1,wherein the grating curves are arranged to be concave shapes against aninput beam coining from a beam input.
 15. The grating coupler of claim1, wherein distances between the grating curves are non-uniform.
 16. Thegrating coupler of claim 15, wherein the distances are arranged todecrease at a fixed rate.
 17. The grating coupler of claim 1, whereinthe distance between a straight end of the first end to a first gratingline is a multiple of the wavelength of the light beam in the waveguide,wherein the multiplier is between 50 and 500.